Super Bug sync signals

kralleman

Member

Donor 2015
Joined
Jan 2, 2015
Messages
157
Reaction score
8
Location
Gteborg, Select a State, Sweden
So I thought I'd try this new subforum out with a bronze age repair-help question. I skimmed through the "First!"-thread started by andrewb and tried to see if there were any decisions or suggestions on what kinds of posts belong here and felt like there are no clear guidelines yet. So let's try with a repair-help question and see how it goes :)

Anyway, I have a Super Bug that is not booting. It feels like the sync signals are off. Attached is a pic from my logic analyzer showing the signals 1H, 2H, 4H, 8H and 16H. The first four of them seem to be ok, but 16H seems to be ticking at roughly the same frequency as 1H. Should it not be twice as long as 8H? I tried using a different probe for the logic analyzer just to rule out a bad probe, no change.

The signals 1H through 16H are developed by the 74161 at N5 (see attached schematic screenshot). I have replaced this IC but 16H is still showing like in the picture. I tested the 7474 at P5 in-circuit with a HP logic comparator and it tested fine. I also made some measures on N6 (which develops signals 32H through 256H) and it seems these signals are off too. I thought I'd start by focusing on the 16H signal from N5 though.

Anyone having any thoughts on where to go next? Any components that would be the "usual suspects"?
 

Attachments

  • 20180419_000454.jpg
    20180419_000454.jpg
    1 MB · Views: 38
  • Screenshot 2018-04-20 14.00.58.png
    Screenshot 2018-04-20 14.00.58.png
    142 KB · Views: 23
Not to be a jerk but are you SURE your 16H probe is on pin 11 of N5? That signal looks like it would be pin 8 & 12 of P5. If you are on the right IC and pin I would check for a short between these two nets. Do you have an oscope? If so scope the 16H signal and see if it looks like a combination of the two signals. (If you see MORE than two levels on the scope then you have two outputs shorted together) The logic analyzer doesn't work well in these situations since it can ONLY show two levels. Your assumptions are absolutely correct, you should see 16H toggling at half the rate of 8H. The fact that the other outputs are correct AND you changed the chip makes me think something else is DRIVING that line and competing with the counter's output. I'm assuming the P on the schematic is +5V? if so I would also make sure the load and clear pins on that counter ARE connected to +5V, If they are floating you may get some weird behavior also.
 
Thanks for the input!

Doublechecked the connections to the logic analyser and they were correct. Probed M5 and N5 with an oscilloscope instead and then the signals looked correct (clean and not mixed and with an expected change in frequency for each output pint). Not sure what the problem is with the logic analyser but I put it aside for the time being. I moved on to probe out the vertical timing signals on the 74161:s at M4 and N4.

On M4 the 4V signal had the same frequency as 8V. TC (pin 15) also looked weak. I piggybacked a new 74161 onto M4 and lifted the 4V leg on that one and it looked more correct. Didn't swap the chip out yet.

On N4 128V (pin 11) was stuck high. Could be due to odd behaviour from M4 I guess. Unfortunately, now I am off for a couple of days of vacation so further troubleshooting will have to wait until next week.
 
I'm not familiar with the game and don't have the schematics, I was just going by the snipet you posted.

For the LA, I've had similar strange behavior if the LA ground probe is unconnected OR not making a good connection to the board ground. In this case since it has no reference it will typically use other signals as the ground reference which will give weird results. I tend to use the scope more for these kinds of problems since I've found ALOT of the bad 74xx parts I've replaced will show some INVALID output voltage like ~2V which a logic probe or analyzer doesn't show.

Assuming M4 doesn't have some strange load logic (If you post the schematic I can confirm this) then 4V and 8V having the same freq is WRONG and M4 is bad. Typically TC (Terminal Count) is used to enable the next counter stage so if it is not outputting correctly then that would explain your 128V problem as well.

Sounds like you found at least 1 problem with the board!
 
I probably should test using another probe for the LA. Anyway, the vertical counters were the ones that were bad. Pulled M4 and replaced it. Then the outputs looked correct and thus could feed correct signals to N4. Scoped N4 outputs and they were bad too. Piggybacked another 74161 on top of it and lifted the output pins on the piggybacked one and then they looked correct. Swapped it and the board fired up correctly. Verified that both 74161 were actually bad with my IC tester. Problem solved! Thanks for the input!
 
Excellent! Odd that both would be bad; who was the manufacturer of those 161's?
 
Back
Top Bottom